Automatic background control



April 29, 1941.

R c. BALLARD AUTOMATIC BACKGROUND CONTROL 7, Filed April 4, 1939 3 Randal l' C. fiazziii ii Patented Apr. 29, 1941 AUTOMATIC BACKGROUND CONTROL Randall G. Ballard, Audubon, N. 1.,

assig-nor to Radio Corporation of America, a corporation of Delaware Application April 4, 1939, Serial No. 265,922

iclaims.

television receiver circuit for reinserting the direct current component of the picture signal at the cathode ray tube and thereby controlling the picture background, this circuit comprising a diode circuit connected across the input circuit of the oathoderay tube. It has been found that while this circuit functions satisfactorily to provide automatic background control, it tends to reduce the high frequency response of the system at the control grid of the cathode ray tube. This is because the diode capacity is connected across the cathode ray tube input electrodes and, therefore, adds to the cathode ray tube input capacity.

It is, accordingly, an object of my invention to provide an improved automatic background control circuit for a television receiver.

It is a further object of my invention to overcome at, least partially the above-mentioned undesirable feature of an automatic background control or D. C. inserting circuit wherein a diode or the like is connected between the last video amplifier and the cathode ray tube.

It is a still further object of my invention to provide an improved D. C. inserting means for either a television transmitter or television receiver.

In practicing my invention, use is made of inductance coils which act to increase the high frequency response both because they resonate a portion of the circuit at a frequency falling just beyond the high frequency end of the desired pass band or otherwise peak the high frequency response and because they isolate the diode capacity from the cathode ray tube input capacity. The invention may also be considered from the standpoint of filter theory, the inductance coils and tube capacities forming a low pass filter, one coil of which is separating the diode capacity from the cathode ray tube capacity. Strictly speaking, however, filter theory is not followed since it is undesirable to add any capacities to those necessarily present even though the terminating impedances may not be correct from a filter standpoint.

The invention will be better understood from the following description taken in connection with the accompanying drawing in which the single figure is a circuit diagram of a portion of a television receiver embodying my invention.

Referring to the drawing, there is shown the last video amplifier tube at l0 which may be a pentode provided with self-bias by means of a cathode resistor II and shunting condenser l2.

The cathode ray tube shown at l3 may be of a well known type comprising a cathode it, a control grid IE, it first anode ii, a second anode l8 and the usual fluorescent screen 59.

The coupling between the plate of the amplifier tube It and the control grid of the cathode ray tube i3 is through an inductance coil 2| and a coupling condenser 22. An operating voltage is applied to the plate of tube l0 through a plate resistor 23 and a second inductance coil 26. Each of the coils 2i and 2d preferably is shunted by a damping resistor.

The direct current inserting circuit comprises a rectifier such as a diode 26 having its cathode connected to the lower end of coil 25 through a condenser 27 and having its pla-te connected to ground. The diode 25 is shunted by a resistor 28.

As will be described hereinafter, a D. C. potential which is a measure of the picture background appears at the cathode of diode 26. This potential is applied to the cathode ray tube grid l6 through a resistor 8i.

The amount of steady bias on the control grid It may be adjusted manually by means of a variable tap 32 on a potentiometer resistor 33. A large bypass condenser 35 holds the cathode H1 at ground potential at the picture frequencies.

From an inspection of the drawing, it will be apparent that the capacity of the diode 26 indicated at 36 is not in parallel with the input capacity of the cathode ray tube I 3 indicated at Bl since these two capacities are separated by the coil 26. The D. C. connection of the diode 26 to the control grid l5, of course, does not place the diode capacity directly across the cathode ray tube input circuit because of the high resistance grid leak resistor iii in the circuit. 7

Theaction of the diode circuit in providing automatic background control is as described in the above-mentioned patent. Briefly, it is as follows: The composite signal comprising both picture signal and synchronizing impulses is applied to the input circuit of amplifier III with such polarity that the synchronizing impulses are negative at the cathode of diode 26. At the transmitter these impulses have been made always to o to "black or to some other fixed level. The synchronizing impulses cause pulses of current to flow through the diode periodically to charge up the condenser 21. By giving resistor 28 a high resistance value, the time constant of the disimminent,

that the condenser 21 discharges only slightly between successive horizontal or line synchronizing impulses whereby the D. C. voltage applied to the grid i8 is a measure of the height of the impulses and, therefore, a measure of the original picture background.

Referring again to the function of the coils 2| and .24, it will be noted that these coils, together with the output capacity 38 of the tube l and the tube capacities 36 and 31, are in the form of a low pass filter where the signal output for the cathode ray tube is taken off the first filter section and the output for the diode 26 is taken off the succeeding section. My circuit arrangement results in the cathode ray tube input capacity 31 being common to the two filter sections, which is desirable since it is larger than either the tube capacity 38 Or 38. Thus the filter theory requirement that capacity 3 be twice capacity 38 and also twice capacity 38 is approximately fulfilled.

Disregarding filter theory, it will be noted that coil II is a series coil resonating with tube capacities 38 and 31 at a frequency higher than the high frequency end of the pass range, while coil 24 acts as a peaking coil for signals applied to the grid I! in addition to providing separation of the capacities I6 and 31.

It has been found that the above-described cir cuit gives the desired automatic background control while making it possible to extend the high frequency end of the amplifier pass range to a point where the high frequency response of the receiver is satisfactory.

From the foregoing, it will be evident that the invention is not limited to television receivers, but is applicable to television transmitters or other apparatus where the tube It may be replaced by an amplifier tube.

I claim as my invention:

1. In combination, an electric discharge tube having a plate electrode, a second electricdischarge tube having a control electrode, said plate electrode being coupled to said control electrode through an inductance coil and a coupling condenser connected in series in the order named, a second inductance coil and a plate resistor connected in series, said second coil being connected to the Junction point of said first coil and said coupling condenser, arectifier coupled through a second coupling condenser to receive the signal voltage appearing across said plate resistor only, a resistor connected in shunt to said rectifier, and a resistor connected between said control electrode and the junctionpoint of said second coupling condenser and the resistor shunting said diode.

charge circuit for condenser 21 is made so large 2. In a television receiver, an electric discharge tube having a plate electrode, a cathode ray tube having a control electrode. said plate electrode being coupled to said control electrode through 'an inductance coil and a coupling condenser connected in series in the order named, a second inductance coil and a plate resistor connected in series, said second coll being connected to the Junction point of said first coil and said coupling condenser, a diode having a plate and a cathode, said diode being coupled to receive the signal a "voltage appearing across said plate resistor only,

the cathode of said diode being connected to the plate end of said resistor through a second coupling condenser, a resistor connected in shunt to said diode, and a resistor connected between said control electrode and said cathode.

3. In a picture channel for carrying picture signals and periodically recurring impulses which are of'greater amplitude than the picture signals, in combination. an electric discharge tube having a plate electrode, a second electric discharge tube having a control electrode, said plate electrode being coupled to said control electrode through an inductance coil and a coupling condenser connected in series in the order named, a second inductance coil and a plate resistor connected in series, said second coil being connected to the Junction point of said first coil and said coupling condenser, rectifier means coupled to receive thesignal voltage appearing across said plate resistor only for deriving a direct current potential which is a measure of the height of said impulses, and a resistor connected between said means and said control electrode for applying said direct current potential thereto.

4. In a television receiver for receiving picture signals and periodically recurring impulses which are of greater amplitude than the picture signals, in combination. an electric discharge tube having a plate electrode, a cathode ray tube having a control electrode, said plate electrode being coupled to said control electrode through an inductance coil and a coupling condenser connected in series in the order named. a second inductance coil and a plate resistor connected in series, said second coil being connected to the Junction point of said first coil and said coupling condenser, means including a diode to receive the signal voltage coupled appearing across said plate resistor only for deriving a direct current potential which is a measure of the height of said impulses, and a resistor connected between said means and said control electrode for applying said direct current potential thereto.

RANDALL C. BALLARD. 

